This application claims Paris Convention priority of DE 100 32 345.6 filed Jul. 4, 2000 the entire disclosure of which is hereby incorporated by reference.
The invention concerns a method of correcting disturbing influences on the MR (=magnetic resonance) signals of a substance disposed in the measuring volume of an MR apparatus excited by one or several RF (=radio frequency) excitation pulses, wherein an RF excitation pulse is irradiated onto the substance and a time-dependent MR signal produced thereby is detected and digitized in a phase-sensitive fashion.
A method of this type is disclosed e.g. by DT 23 52 315 C2 and EP 0 522 191 A1.
MR field stabilization should be as accurate as possible to keep the magnetic field as constant as possible. Towards this end, both small interfering amplitudes which produce small frequency changes as well as relatively large external magnetic interfering amplitudes should be compensated. Field instabilities have a disturbing effect, in particular, for prolonged experiments which can take several hours. Even with the most modern superconducting magnets, the magnetic field changes with time due to a very small but important electrical resistance of the coil itself and due to external influences, such as the surrounding temperature.
To compensate for field inhomogeneities, an MR signal is conventionally used as a reference in a control circuit, wherein the shape of the dispersion signal is utilized. The amplitude in the center of the signal is zero and that amplitude is non-zero on both sides of the center. Most importantly, the sign of the signal changes during passage through resonance. If the initially detected signal is detected exactly on resonance and the magnetic field then changes, a signal is produced in a detector. The sign of the signal indicates the direction of change of the magnetic field. In a superconducting magnet, the so-called lock signal is amplified and fed to a coil which produces a magnetic field to either increase or decrease the main magnetic field. In an electromagnet, the lock signal controls the current supplied to the magnet. This method is relatively inaccurate since only a small portion of the signal is used to control the magnetic field. Exact determination of the passage of the signal through resonance is also difficult.
DT 23 52 315 C2 discloses a method of stabilizing the relationship between measuring frequency and magnetic field strength in a spin resonance spectrometer. The spin resonances of a standard substance introduced into the magnetic field of the spin resonance spectrometer are excited by RF pulses having a dense frequency spectrum. The frequency of the resonance signal of the standard substance, contained in the pulse response signal, is determined and compared with a reference frequency. The determined deviation provides a controlled variable for adjusting the magnetic field strength or the measuring frequency, with the frequency of the resonance signal of the standard substance being determined through Fourier analysis of the pulse response signal. The spin resonances of the standard substance are adjusted, after sufficient approximation of the relationship between measuring frequency and magnetic field strength, to the desired relationship using a continuous-wave signal to produce a suitable resonance signal for the standard substance. Such a pulsed control is very slow and therefore inaccurate.
EP 0 522 191 A1 discloses a method of compensating time-dependent field disturbances in magnetic fields of electromagnets having high field homogeneity. In particular, a dispersion nuclear signal ux of a comparative substance, located in the sample region of the superconducting electromagnet within which the magnetic resonance is measured, is recorded and used for compensation by producing a current in a field correction coil of the electromagnet in dependence on the dispersion signal. The absorption signal uy of the comparative substance is also recorded and compensation is carried out in dependence on the variable ux/uy and/or (1/uy) (dux/dt). This prior art method is also too slow for field stabilization required for a prolonged experiment.
It is therefore the underlying purpose of the present invention to realize a method for faster and more exact control, i.e. a method which decreases the sampling time and increases control sensitivity.
This object is achieved in accordance with the invention in a simple but effective fashion in that a time dependence si(ti) of the detected and digitized MR signal is used to determine a time dependence xcex94xcfx86i(ti) of the phase of the MR signal relative to a predetermined reference phase xcfx86refi(ti) of a reference signal srefi(ti). The determined phase is then digitized and one or more correction or control values are determined therefrom.
In the inventive method, the phase dependence is the actually relevant variable for a deviation of the magnetic field from the desired value. In prior art, most of the measuring points play no or only a subordinate role in determining the magnetic field deviation. However, the values of a free induction decay (FID) at later times of the FID are of particular interest for control and correction. Advantageously, in the inventive method, nearly all measuring points are used for determination of the magnetic field deviation and thus for control. This guarantees considerably improved control accuracy and furthermore permits faster repetition with less measuring points for equal or improved control quality. Control is possible with means which are already present in an MR apparatus. Additional electronic equipment for controlling the frequency or the magnetic field is not required.
In a particularly preferred variant, the average slope of the phase of the MR control signal, relative to the reference phase, is determined by means of a linear regression through the corresponding time function xcex94xcfx86i(ti). The use of a straight line permits estimation of a previous frequency deviation, wherein the influence of noise on the estimate is suppressed by the straight line fit.
Another variant is characterized in that a curvature of the time function xcex94xcfx86i(ti) is determined and used for predicting the probable behavior of the system with respect to nominal deviations in the immediate future. Determination of the curvature further improves the control or correction since extrapolation permits prediction of the deviation from the ideal case, thereby producing adaptive and more accurate control.
In a further development, the curvature of the time function xcex94xcfx86i(ti) is determined by means of a polynomial fit of higher order. A polynomial fit permits particularly accurate approximation of the time function. Accurate determination of the actual frequency is thus possible. This is particularly important at the end of an FID.
A further method variant is characterized in that the contributions to the correction or controlled variable at different times ti from individual measuring points of the phase xcex94xcfx86i, relative to the reference phase, are weighted in dependence on the signal-to-noise-ratio. Stronger weighting of low-noise increases the control accuracy. It is, however, also feasible to weight in dependence on the signal amplitude. Signals having a large amplitude are thereby used to a larger extent for determination of the controlled or correction variable than are signals having smaller amplitudes.
In a further method variant, RF excitation pulses are irradiated onto the substance in the measuring volume at pulse angles of xcex1 less than  less than 90xc2x0. This approach does not require waiting for establishment of an equilibrium to permit rapid determination of the controlled variables and high repetition rates, thereby increasing the speed of the method.
If the frequency of the reference signal is chosen to be different than the desired frequency and if this preferably constant deviation is taken into consideration for determining the controlled or correction variable, summing up of residual signals from previous excitations is prevented.
It is particularly preferred to subject the detected MR signal to a digital filtration, preferably a bandpass filtration, which eliminates high-frequency and low-frequency disturbances and permits more accurate determination of the controlled or correction variable.
A method variant is characterized in that deviations of the magnetic field from a desired value are corrected or readjusted by means of the correction or controlled variable to stabilize the magnetic field in the measuring volume of the MR apparatus. In this fashion, as in prior art, the correct physical variable is controlled, however, with different correction variables. Control is thereby simplified. When the magnetic field is controlled, the measured frequencies remain almost constant.
In a particularly preferred method variant, all frequencies of the MR apparatus used for signal extraction, including the excitation frequency, are corrected or readjusted by means of the correction or controlled variable. In this case, the frequencies which are used for signal excitation or detection are tracked. The magnetic field can therefore take its course. The systematic error which occurs by controlling the frequencies and not the magnetic field, can be neglected. Since the magnetic field must not be controlled, one coil can be omitted in the MR apparatus. This operation with one less coil reduces energy requirements.
In a preferred variant of the inventive method, the method steps are repeated several times, preferably periodically with one frequency deviation Afi being determined for each repetition as the instantaneous controlled variable. In this fashion, it is possible to control the magnetic field for the duration of the repetitions, i.e. the magnetic field is continuously controlled.
In a particularly preferred variant of the method, the time dependence of the frequency deviation xcex94fi(ti) for all measuring points is determined at the various times ti from xcex94f(t)=[xcex94xcfx86i(ti)xe2x88x92xcex94xcfx86i(0)]/ti. These values can be used to predict the future frequency deviations and the controlled or correction variable can be correspondingly determined.
A further development consists in that the controlled variable is determined from the frequency deviations of all measuring points as an average value  less than xcex94fi greater than . Averaging of the frequency deviations produces a relatively precise controlled variable which is easy to use. Determination of this controlled variable is simple, which permits faster control.
In one embodiment, the substance disposed in the measuring volume of the MR apparatus comprises a sample substance and a control substance. A suitable control substance has only a few dominant lines. These sharp lines permit exact control, which would not necessarily be the case if control had to be carried out with the sample substance which may comprise several poorly defined frequency components. Superposition of frequency components causes large variations in the frequency amplitudes which could produce calculation errors in determining the frequencies. This would be a problem, in particular, when comparing successive scans. However, using the sample substance alone has considerable advantages in many cases.
A further embodiment of the inventive method is characterized in that, for correcting disturbances during detection and digitization of the MR signal of the sample substance, the MR signal of the control substance is detected and digitized at the same time as the MR signal of the sample substance is detected, and a time dependence xcex94xcfx86(t) of the phase of the MR signal of the control substance, relative to the phase xcfx86ref of a predetermined reference signal, is determined from the time dependence s(t) of the MR signal of the control substance, and for each digitized point the detected MR signal of the sample substance is readjusted according to the phase xcex94xcfx86(t), while taking into consideration the gyromagnetic ratio of sample substance and control substance. Since the control signal is recorded at the same time as the sample signal, it is possible to correct frequency shifts within a scan as well as the frequency and phase position. In contrast to prior art, correction occurs synchronously. A distorted signal can therefore be measured and subsequently be precisely corrected. Moreover, the phase can be corrected at each recorded point, in particular, before the start of the next excitation.
In a preferred further development of the inventive method, the time dependence xcex94xcfx86(t) of the phase of the detected MR signal of the control substance, relative to the phase of the reference signal, is subjected to a smoothing process before readjustment of the MR signal of the sample substance. This suppresses noise and improves the quality of the control or correction process.
In a further variant for correcting disturbances occurring in an MR signal recording sequence with which the magnetic fields are systematically varied and with which certain magnetic field configurations are successively repeated in the measuring volume during the MR recording sequence, the MR signal of the control substance is excited and detected while one or more of these magnetic field configurations is present in the measuring volume. The control signals are produced at times when the magnetic field of the MR apparatus changes in a directed or intentionally equivalent fashion. Whereas prior art requires switching off control of the magnetic field (lock) or of the frequency for directed magnetic field changes, this measure permits control of the frequency for variable magnetic fields.
In particular for measuring sequences of in-vivo measurements not having a control substance, a sample signal can be measured and evaluated in accordance with the invention at similar repetitive time intervals, preferably between excitation of the measuring sample signal and its first phase encoding. In this manner, the frequency of the useful signal (usually an echo) can be corrected following the measuring sequence and/or the drift of the magnetic field B0. It is possible to produce one additional control signal at each selected point of the sequence.
In a further development of the method, the MR recording sequence is sorted in accordance with successive scan regions with comparable properties and corresponding signal points are compared for correcting disturbances. Since only similar signals are compared, this is only possible when the overall sequence is repeated.
In an alternative further development, the predetermined reference signals are taken from a reference scan, in particular, from the first scan of the MR recording sequence. Instead of theoretical generation of the reference signals, which is almost impossible with systematic variation of the magnetic fields, the reference points for correction of disturbances are produced in a reference scan.
In a preferred fashion, the reference scan is selected such that the phase change is as small as possible with respect to preceding and following scans. If the phase change is relatively small with respect to neighboring scans, one can assume that the phase change is also small at the time at which the reference scan is recorded. The reference scan is thereby selected at a favorable point in time to permit exact control.
It is particularly advantageous to predetermine and fixedly program the reference signals for a certain MR recording sequence. The recording sequences are often repeated. Once a good reference scan has been found and programmed it can be used for many future recording sequences.
In a further method variant in accordance with the invention, several MR signals are recorded and combined, in particular accumulated, and post-correction of the individual MR signals is carried out before combination, in particular before accumulation. The scans are recorded and corrected in accordance with the determined frequency error. Only after this correction are the scans further processed. If e.g. the FIDs are added after correction, the FIDs have a larger area than that obtained by adding uncorrected FIDs.
The area below an FID is a measure of the correction quality. This method is especially useful if real-time correction is not possible, as is e.g. the case when the MR apparatus electronics are inaccessible. It is also feasible to reuse the sum of all corrected FIDs or scans as a reference scan. This is advantageous since correction is further improved due to the improved signal-to-noise ratio. Furthermore, an additional phase correction can follow the frequency correction of the FIDs. In this case as well, the sum of the frequency-corrected FIDs can be used as a reference.
Advantageously, a time dependence of each corresponding phase signal xcfx86i(ti) is determined from the time dependence si(ti) of the associated MR signals, wherein a phase signal or an average value thereof is selected from several phase signals as a reference phase signal xcfx86refi(ti), wherein the MR signals are phase-corrected by means of this reference phase signal xcfx86refi(ti). Phase correction occurs in addition to frequency correction and each point can be corrected separately.
If the reference phase signal xcfx86refi(ti) is determined from an MR signal which differs as little as possible from the neighboring MR signals, a particularly good reference signal is selected since one must assume that the reference signal is selected within a measuring range in which the magnetic field or the frequency changes only slightly.
Phase correction of MR signals is preferably applied in an MR spectroscopy method which does not have magnetic field locking, in particular in an in-vivo spectroscopy method. Correction is possible without intervention in the MR apparatus.
In a further aspect of the invention, the above mentioned object is achieved by an MR apparatus for carrying out the inventive method, comprising a magnet system for producing a magnetic field in a measuring volume, comprising means for irradiating an RF excitation pulse onto a control or sample substance in the measuring volume and producing one or more MR signals, comprising means for phase-sensitive detection of the MR signal or the MR signals, and with a control device for stabilizing the magnetic field having means for detecting the generated MR signal or MR signals and for deriving a correction or controlled variable to correct deviations of the magnetic fields or of the frequency from a predetermined desired value, wherein the control device also comprises a calculating unit for determining and digitizing, from the time dependence si(ti) of a detected MR signal, a time dependence xcex94xcfx86i(ti) of the phase of the MR signal relative to the phase xcfx86refi(ti) of a predetermined reference signal srefi(ti) and for determining one or more correction or controlled variables therefrom.
Such an MR apparatus permits control or correction of the magnetic field or of the frequencies in real time. As a result, data must not be intermittently stored prior to correction, since correction is possible before termination of the measurement. This saves memory space and time.
In a preferred embodiment, the calculation unit may initiate detection of the MR signal of the control substance synchronously with detection of the MR signal of the sample substance as well as digitization of the recorded signals, and readjusts, for each digitization point, the detected MR signal from the sample substance in correspondence with the phase xcex94xcfx86i(ti) while taking into consideration the gyromagnetic ratio between the sample substance and the control substance. This permits later correction of a recorded MR signal of a sample substance using correction variables determined from an MR signal of a control substance recorded synchronously thereto.
In a preferred further development, the calculation unit can use the time dependence si(ti) of the MR signals to determine the time dependence of the respective phase signals xcfx86i(ti), can select such a phase signal or an average value from several such phase signals as a reference phase signal xcfx86refi(ti), and can phase-correct the MR signals before accumulation thereof using this reference phase signal xcfx86refi(ti). Later correction of the phase in addition to the frequency is thereby possible.
A further development concerns a calculating unit for use in the above-mentioned MR apparatus which permits real-time processing of the data.
Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below can be used in accordance with the invention either individually or collectively in any arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration, rather have exemplary character for describing the invention.
The invention is shown in the drawing and is explained in more detail by means of embodiments.